Basement tub strange drain

KenS85April 25, 2013

I'm renovating a basement apartment of a building I bought. Today I realized there was a 5 or so inch hole in the floor with a cap on it. I opened it up and there's 8 inch or so of water sitting in there. When I turn on the water in the tub the hole fills up higher but always at least an inch or 2 from over flowing. Anyone know what this is for? Do I need to be concerned with it backing up or over flowing? See the picture to better understand. Thanks.